Effect of Milk Constituents on Hepatic Cholesterogenesis

Abstract

Utilization of the liver slice technique enabled the isolation of two inhibitory preparations from bovine skim milk which were active in reducing hepatic cholesterol biosynthesis. One of the inhibitors was in the dialyzable fraction and was identified as orotic acid (OA). The other inhibitor, present in the nondialyzable fraction, was not exclusively identified. Preliminary data indicated that it was of high molecular weight, polar, and was neither classical protein nor lipid. This material migrated as one band in polyacrylamide disc gel electrophoresis. Alkaline hydrolysis followed by formation of the methyl esters, GLC and mass spectral analyses indicated that the fraction contained nitrogenous compounds of high molecular weight. Orotic acid appeared to act primarily by inhibiting cholesterol biosynthesis before the formation of mevalonic acid; whereas the nondialyzable inhibitor exerts its effect beyond the formation of mevalonic acid in the biosynthetic pathway. Human milk was also found to inhibit the incorporation of both labeled acetate and mevalonic acid into cholesterol by the liver. Orotic acid could not be the inhibitor in human milk since the samples employed in the study did not contain this compound. Administration of OA-6-14C to rats results in its conversion to uracil by the liver. Subsequent work demonstrated that uracil exerted an inhibitory action on hepatic cholesterol biosynthesis similar to that of OA when both were incubated with rat liver slices.